Cam Suggested Compression Ratios..

General engine tech -- Drag Racing to Circle Track

Moderator: Team

BlackoutSteve
Expert
Expert
Posts: 812
Joined: Wed Jan 18, 2006 6:53 am
Location: Melbourne, Australia.
Contact:

Cam Suggested Compression Ratios..

Post by BlackoutSteve » Wed Feb 20, 2013 2:59 am

I am wondering how strongly we need to adhere to the camshaft manufacturer's suggested compression ratios.
I am swapping cylinder heads and if I keep the same pistons, I will lose 0.85 of a point -from 12:1 to 11.15:1 due to the larger chambers of the new heads.
The cam is a Crane 138801, and it states a minimum of 12:1 which will require a new set of pistons if I want to maintain that 12:1.
http://www.cranecams.com/product/cart.p ... il&p=24638

Will it be OK to drop nearly a full point with this cam?

Here are two cams that are almost identical..
My old Crane flat tappet 134781 that requires 11-12:1 comp. http://www.cranecams.com/product/cart.p ... il&p=24459
..and a Comp Cams 11-219-4 http://www.compcams.com/Company/CC/cam- ... d=458&sb=2

Both cams have 248° Intake with the Crane 10° more Exh. -all at 0.050".
Both have a 110° LSA and about the same lift..
Why does the Comp only need 10:1 and the Crane 1-2 full points more for roughly the same camshaft?
Is it all just where the intake valve closes?
Post pictures, or it didn't happen!

Belgian1979
Guru
Guru
Posts: 3437
Joined: Sat Mar 26, 2011 11:34 am
Location: Belgium - Koersel

Re: Cam Suggested Compression Ratios..

Post by Belgian1979 » Wed Feb 20, 2013 3:05 am

My understanding is that you need to compare DCR's of the different cams on the engine to see what the effect will be. A CR drop will mainly affect low speed torque. At higher rpm, your engine might see the actual static CR, which is determined by the octane rating of the fuel you use.

So in essence you use as high a static CR as doable with the fuel you run and the type of heads. Put in the cam numbers and see where you end up on DCR. If the DCR is around 8-8.5 you're golden (if this is street gas). If you have got everything else under controle you might go a tad higher (cooling, quench, flame travel)

pdq67
Guru
Guru
Posts: 6445
Joined: Thu Mar 04, 2010 8:05 pm

Re: Cam Suggested Compression Ratios..

Post by pdq67 » Wed Feb 20, 2013 5:23 am

I used to use this with a grain of salt.

Advertised duration cam families vs static compression ratio's here.

248/250/252 = 8 to 1;

258/260/262 = 9 to 1; (The 266 can go either way).

268/270/272 = 10 to 1; (Same with the 274 can go either way).

278/280/282 = 10.5 to 1; (Same with the 284).

288/290/292 = 11 to 1; (Same with the 294/296).

and

300 = 12 to 1.

Seemed to work pretty good, but these are for the older cams.

There are a whole bunch of things that affect this such as; aluminum head vs iron, camber shape and type; quench distance and headgasket thickness; of course fuel octane; timing; etc., etc...

pdq67

Cubic_Cleveland
Guru
Guru
Posts: 1111
Joined: Thu Feb 23, 2012 11:44 pm
Location: Australia

Re: Cam Suggested Compression Ratios..

Post by Cubic_Cleveland » Wed Feb 20, 2013 6:25 am

Is it possible to mill the new heads to gain back compression, or are you running out of piston to valve clearance?

rfoll
Guru
Guru
Posts: 2053
Joined: Mon Jan 11, 2010 12:44 pm
Location: St. Helens, OR

Re: Cam Suggested Compression Ratios..

Post by rfoll » Wed Feb 20, 2013 10:58 am

How big is the engine? That cam does not look particularly large for a 454. I can't imagine it being too big for a 11:1+ BBC.
So much to do, so little time...

User avatar
CamKing
Guru
Guru
Posts: 8263
Joined: Wed Aug 08, 2007 4:05 pm
Location: Denver, NC
Contact:

Re: Cam Suggested Compression Ratios..

Post by CamKing » Wed Feb 20, 2013 2:09 pm

Those catalog cam "compression requirements" are useless as teets on a boar, and based off of nothing but some 1960's rule of thumb.
Mike Jones
Jones Cam Designs
Denver, NC
jonescams@bellsouth.net
http://www.jonescams.com
(704)489-2449

avengerengines
Expert
Expert
Posts: 844
Joined: Fri Oct 01, 2010 4:07 pm
Location: West Michigan

Re: Cam Suggested Compression Ratios..

Post by avengerengines » Wed Feb 20, 2013 2:18 pm

CamKing wrote:Those catalog cam "compression requirements" are useless as teets on a boar, and based off of nothing but some 1960's rule of thumb.
=D>

F-BIRD'88
Guru
Guru
Posts: 5722
Joined: Tue Mar 27, 2007 6:56 pm
Location: Ontario, Canada

Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 » Wed Feb 20, 2013 2:24 pm

The cr "requirement" is a guide line.
It will run fine at 11:1cr. based on just the cr change it will now make about 3% less power than before.

If the new heads flow better then the old heads you may end up with more net power than before.
Even with less compression ratio.

You can play with the installed position of the cam a bit to find the sweet spot for power and torque.
If it was a good cam before it will still be a good cam.

Cams and cr.... every cam in every engine will make a bit more power with 12:1 cr,, than 11:1
and even more power at 13:1cr
The difference is going to be approx 3 to 4 % per compression ratio change.

run it.

Crepitus
New Member
New Member
Posts: 44
Joined: Tue Apr 03, 2012 3:09 pm
Location: Central WA 2 hours from n e where

Re: Cam Suggested Compression Ratios..

Post by Crepitus » Wed Feb 20, 2013 5:20 pm

CamKing wrote:Those catalog cam "compression requirements" are useless as teets on a boar, and based off of nothing but some 1960's rule of thumb.
Awww crap, just when I thought I found the EASY button. #-o
Rember when racing was dangrous and sexx was safe?

Dodge Freak
Guru
Guru
Posts: 1712
Joined: Sun Oct 21, 2007 6:56 pm

Re: Cam Suggested Compression Ratios..

Post by Dodge Freak » Wed Feb 20, 2013 6:12 pm

I have an old NOS Blazer cam, mechanical, 300/310 adv...238/248@.050, thinking of using in a 340 SBM with only 10 to 1 compression, maybe 9.75 to 1 ?

is there a rule of thumb of min cranking compression so I could check it before buttoning it up, I do have another hyd cam a bit smaller--the old "Hemi" 284 cam

Crepitus
New Member
New Member
Posts: 44
Joined: Tue Apr 03, 2012 3:09 pm
Location: Central WA 2 hours from n e where

Re: Cam Suggested Compression Ratios..

Post by Crepitus » Wed Feb 20, 2013 6:37 pm

Dodge Freak wrote:I have an old NOS Blazer cam, mechanical, 300/310 adv...238/248@.050, thinking of using in a 340 SBM with only 10 to 1 compression, maybe 9.75 to 1 ?

is there a rule of thumb of min cranking compression so I could check it before buttoning it up, I do have another hyd cam a bit smaller--the old "Hemi" 284 cam
At 238/248 I would guess its a hot street. 92 oct, aluminum head, good tune, 190-205 would be good under 180 will be a dog.
Rember when racing was dangrous and sexx was safe?

F-BIRD'88
Guru
Guru
Posts: 5722
Joined: Tue Mar 27, 2007 6:56 pm
Location: Ontario, Canada

Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 » Wed Feb 20, 2013 6:37 pm

Dodge Freak wrote:I have an old NOS Blazer cam, mechanical, 300/310 adv...238/248@.050, thinking of using in a 340 SBM with only 10 to 1 compression, maybe 9.75 to 1 ?

is there a rule of thumb of min cranking compression so I could check it before buttoning it up, I do have another hyd cam a bit smaller--the old "Hemi" 284 cam
it will run very well This cam is still sold as the F-238/3200-2-14 cam. It is not "old" at all. Degree it in so the intake closes @46 deg ABDC @ .050" lifter rise.
needs a 3000-3500 stall or 4 speed and some 4.10's 4.30's in a Dart or Duster.

The valve lash is .022" hot.. try setting the valve lash cold at .018". A lot easier and pain free that way and you will be more likely to take care of periodic lashing with a pain free method.
The net lash opens up a bit when it gets warmed up.

real good "nitrous cam" for that lil 340. GOT NITROUS?!

on a low compression ratio engine (7.5:1 to 8.5:1cr) this cam makes a real good street strip supercharger cam in a 340 360. Or even a 318. A good match to a Weiand 142 blower on a low cr 340.
Last edited by F-BIRD'88 on Wed Feb 20, 2013 7:08 pm, edited 3 times in total.

Strange Magic
Guru
Guru
Posts: 1721
Joined: Sun Dec 03, 2006 1:14 am
Location: New York
Contact:

Re: Cam Suggested Compression Ratios..

Post by Strange Magic » Wed Feb 20, 2013 6:51 pm

Why would you want to reduce the compression (torque output/broadness) of the engine by dropping it almost a point of compression? Is the purpose to rid the engine of cylinder pressure, or is it for the purpose to change fuels to save money? Just curious.

In many cases, the suggested items listed in a camshaft catalog are non-sense. In reality, many should read as so:

Poor low end power, poor mid range power, adequite top end power. I other words, this is the wrong cam for your engine so we want you to band aid the lack of cylinder pressure which yields poor velocity at low and mid rpms, with additional rear end gear ratio and additional converter stall. The lighter your car can be, the less disasterous this camshaft will appear.

The below means nothing and is absolutely useless without having an understanding of LSA and intake install. It also is useless information unless you understand the size of the pump and the piston speed, of which is determined by bore and stroke, and rod length and stroke.
I used to use this with a grain of salt.

Advertised duration cam families vs static compression ratio's here.

248/250/252 = 8 to 1;

258/260/262 = 9 to 1; (The 266 can go either way).

268/270/272 = 10 to 1; (Same with the 274 can go either way).

278/280/282 = 10.5 to 1; (Same with the 284).

288/290/292 = 11 to 1; (Same with the 294/296).

and

300 = 12 to 1.
strangemagicperformance.com
Strange Magic Camshaft Technologies
Decisions on parts and advise should not be based on how much money a company can pour into marketing. This is a common mis-conception in the industry.

F-BIRD'88
Guru
Guru
Posts: 5722
Joined: Tue Mar 27, 2007 6:56 pm
Location: Ontario, Canada

Re: Cam Suggested Compression Ratios..

Post by F-BIRD'88 » Wed Feb 20, 2013 7:01 pm

Strange Magic wrote:Why would you want to reduce the compression (torque output/broadness) of the engine by dropping it almost a point of compression? Is the purpose to rid the engine of cylinder pressure, or is it for the purpose to change fuels to save money?
It is real simple and just this simple... He is not looking to drop the compression ratio. But the new heads have a larger combustion chamber than the old ones .
This is not going to kill this motor. It will still have lots of cylinder pressure, torque and power.
If the heads heads are that much better he may well end up with more net power.

You can play with the cam installed position a bit +/- to good effect to find the new sweet spot.
The engine will show you what it likes best . Do not worry about the cranking pressure.
The only thing that matters is how it runs down the track.

If the cam was a good cam before, it will still be a good cam>>

Strange Magic
Guru
Guru
Posts: 1721
Joined: Sun Dec 03, 2006 1:14 am
Location: New York
Contact:

Re: Cam Suggested Compression Ratios..

Post by Strange Magic » Wed Feb 20, 2013 7:09 pm

It is real simple and just this simple... He is not looking to drop the compression ratio. But the new heads have a larger combustion chamber than the old ones .
F-Bird, I clearly understand what he wrote. I am asking him, the topic starter why he would do this or settle for this. I've read through his posts, not interested in anyone elses posts. I would like to hear this from him, rather than others taking pop shots at what might be going on in his own mind.
strangemagicperformance.com
Strange Magic Camshaft Technologies
Decisions on parts and advise should not be based on how much money a company can pour into marketing. This is a common mis-conception in the industry.

Post Reply